1. Field of the Invention
The present invention relates to a thermal transfer film comprising a substrate film and a sublimable dye layer, a hot-melt ink layer, and optionally a transferable protective layer region provided in parallel to one another on the substrate film. More particularly, the present invention relates to a thermal transfer film which can form durable images on an image-receiving object to provide an object, such as an identification (ID) card, having a transferred photograph-like image together with transferred information represented by letters, symbols and the like.
2. Background Art
Various thermal transfer recording systems are known in the art. One of them is a sublimation-type thermal recording system wherein a transfer sheet comprising a substrate film and, provided thereon, a dye layer containing a sublimable dye and a binder is prepared and the dye contained in the dye layer is sublimated (thermally transferred) by means of a thermal head, a laser beam, or the like in response to image information to carry out recording.
A hot-melt-type thermal recording system is also known wherein a thermal transfer film comprising a substrate film and, provided thereon, a hot-melt ink layer containing a colorant, such as a pigment, and a vehicle, such as wax, is prepared and the hot-melt ink layer is heated by the same heating means as described above to soften and transfer components of the ink layer to form an image.
The thermal transfer recording systems enable various images to be simply formed and, hence, have become utilized in prints wherein the number thereof may be relatively small, for example, in the preparation of identification (ID) cards and the like.
The preparation of ID cards and the like using the hot melt type thermal transfer film described just above is disadvantageous in that the formation of gradational images, such as a photograph-like image of a person's face, is difficult although monotonous images, such as letters and numerical values, can be easily formed.
On the other hand, the use of the sublimation dye transfer-type thermal transfer film can provide excellent gradational images such as a photograph-like image of a person's face. Images of letters, symbols, and the like formed by this thermal transfer film, however, lack in density and sharpness, making it impossible to form characters and bar codes, for OCR, readable by infrared radiation.
The use of the hot-melt-type thermal transfer film in combination with the sublimation-type thermal transfer film is considered as a method for solving the above problems. This method, however, involves a complicated operation. For this reason, as disclosed in Japanese Patent Laid-Open No. 453905/1991, a thermal transfer film has been developed which comprises a continuous substrate film and a sublimation-type dye layer and a hot-melt-type ink layer provided in parallel to each other on the substrate film. For this thermal transfer film, individual ink coating regions are formed to have the same area. Therefore, when an ID card with a portion for a photograph-like image of a person's face and a portion for information represented by letters, such as company names or personal names, occupying only a small proportion of the card is prepared using this thermal transfer film, problems occur such as prolonged printing time and high running cost.
In order to solve these problems, Japanese Patent Laid-Open No. 281989/1989 discloses a thermal transfer ink sheet comprising a sublimable dye layer region having a relatively smaller area and a hot-melt ink layer having a relatively larger area.
The hot-melt ink layer described in the above laid-open document, however, is a wax-based ink layer which has poor durability in respect of abrasion resistance, plasticizer resistance, and the like. The poor durability leads to problems of the disappearance of information represented by letters recorded together with a photograph-like image during use for a long period of time and the forgery of ID card because the information is easily rewritable.
Further, there are additional problems including that when an ID card is repeatedly put in and out of a purse or a card case, blurring of the photograph-like image of a person's face or fading occurs and that the image is blurred during use of an ID card for a long period of time due to an influence of a plasticizer or the like contained in a polyvinyl chloride card case.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer film which can simply form, in a short time and in a cost-effective manner, a gradational image and a monotonous image which can effectively prevent the forgery of recorded information.